Spatial and temporal variability and long-term trends in skew surges globally

Storm surges and the resulting extreme high sea levels are among the most dangerous natural disasters and are responsible for widespread social, economic and environmental consequences. Using a set of 220 tide gauges, this paper investigates the temporal variations in storm surges around the world and the spatial coherence of its variability. We compare results derived from two parameters used to represent storm surge: skew surge and the more traditional, non-tidal residual. We determine the extent of tide-surge interaction, at each study site, and find statistically significant (95% confidence) levels of tide-surge interaction at 59% of sites based on tidal level and 81% of sites based on tidal-phase. The tide-surge interaction was strongest in regions of shallow bathymetry such as the North Sea, north Australia and the Malay Peninsula. At most sites the trends in the skew surge time series were similar to those of non-tidal residuals, but where there were large differences in trends, the sites tended to have a large tidal range. Only 13% of sites had a statistically significant trend in skew surge, and of these approximately equal numbers were positive and negative. However, for trends in the non-tidal residual there were significantly more negative trends. We identified 8 regions where there were strong positive correlations in skew surge variability between sites, which meant that a regional index could be created to represent these groups of sites. Despite strong correlations between some regional skew surge indices, none were significant at the 95% level, however, at the 80% level there was significant positive correlation between the north-west Atlantic—south and the North Sea. Correlations between the regional skew surge indices and climate indices only became significant at the 80% level, where Ni?o 4 was positively correlated with the Gulf of Mexico skew surge index and negatively correlated with the east Australia skew surge index. The inclusion of autocorrelation in the calculation of correlation greatly reduced their significance, especially in the short time-series used for the regional skew surge indices. Skew surge improved the representation of storm surge magnitudes, and therefore allows a more accurate detection of changes on secular and inter-annual time scales

Global secular changes in different tidal high water, low water and range levels

Tides exert a major control on the coastal zone by influencing high sea levels and coastal flooding, navigation, sediment dynamics, and ecology. Therefore, any changes to tides have wide ranging and important implications. In this paper, we uniquely assess secular changes in 15 regularly used tidal levels (five high water, five low water and five tidal ranges), which have direct practical applications. Using sea level data from 220 tide gauge sites, we found changes have occurred in all analyzed tidal levels in many parts of the world. For the tidal levels assessed, between 36% and 63% of sites had trends significantly different (at 95% confidence level) from zero. At certain locations, the magnitude of the trends in tidal levels were similar to trends in mean sea level over the last century, with observed changes in tidal range and high water levels of over 5 mm/yr and 2 mm/yr, respectively. More positive than negative trends were observed in tidal ranges and high water levels, and vice versa for low water levels. However we found no significant correlation between trends in mean sea level (MSL) and any tidal levels. Spatially coherent trends were observed in some regions, including the north-east Pacific, German Bight and Australasia, and we also found that differences in trends occur between different tidal levels. This implies that analyzing different tidal levels is important. Because changes in the tide are widespread and of similar magnitude to MSL rise at a number sites, changes in tides should be considered in coastal risk assessments

Editorial: Sea level variability and change

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A geological perspective on potential future sea-level rise

During ice-age cycles, continental ice volume kept pace with slow, multi-millennial scale, changes in climate forcing. Today, rapid greenhouse gas (GHG) increases have outpaced ice-volume responses, likely committing us to > 9 m of long-term sea-level ri

Improving Estuarine Flood Risk Knowledge through Documentary Data Using Multiple Correspondence Analysis

Estuarine margins are usually heavily occupied areas that are commonly affected by compound flooding triggers originating from different sources (e.g., coastal, fluvial, and pluvial). Therefore, estuarine flood management remains a challenge due to the need to combine the distinct dimensions of flood triggers and damages. Past flood data are critical for improve our understanding of flood risks in these areas, while providing the basis for a preliminary flood risk assessment, as required by European Floods Directive. This paper presents a spin-off database of estuarine flood events built upon previously existing databases and a framework for working with qualitative past flood information using multiple correspondence analysis. The methodology is presented, with steps ranging from a spin-off database building process to information extraction techniques, and the statistical method used was further explored through the study of information acquired from the categories and their relation to the dimensions. This work enabled the extraction of the most relevant estuarine flood risk indicators and demonstrates the transversal importance of triggers, since they are of utmost importance for the characterization of estuarine flood risks. The results showed a relation between sets of triggers and damages that are related to estuarine margin land use, demonstrating their ability to inform flood risk management options. This work provides a consistent and coherent approach to use qualitative information on past floods, as a useful contribution in the context of scarce data, where measured and documentary data are not simultaneously available.info:eu-repo/semantics/publishedVersio

Mean sea level variability in the North Sea: Processes and implications

Mean sea level (MSL) variations across a range of time scales are examined for the North Sea under the consideration of different forcing factors since the late 19th century. We use multiple linear regression models, which are validated for the second half of the 20th century against the output of a tide+surge model, to determine the barotropic response of the ocean to fluctuations in atmospheric forcing. We find that local atmospheric forcing mainly initiates MSL variability on time scales up to a few years, with the inverted barometric effect dominating the variability along the UK and Norwegian coastlines and wind controlling the MSL variability in the south from Belgium up to Denmark. On decadal time scales, MSL variability mainly reflects steric changes, which are largely forced remotely. A spatial correlation analysis of altimetry observations and gridded steric heights suggests evidence for a coherent signal extending from the Norwegian shelf down to the Canary Islands. This fits with the theory of longshore wind forcing along the eastern boundary of the North Atlantic causing coastally trapped waves to propagate over thousands of kilometers along the continental slope. Implications of these findings are assessed with statistical Monte-Carlo experiments. It is demonstrated that the removal of known variability increases the signal to noise ratio with the result that: (i) linear trends can be estimated more accurately; (ii) possible accelerations (as expected, e.g., due to anthropogenic climate change) can be detected much earlier. Such information is of crucial importance for anticipatory coastal management, engineering, and planning

KwaZulu-Natal coastal erosion events of 2006/2007 and 2011: A predictive tool?

Severe coastal erosion occurred along the KwaZulu-Natal coastline between mid-May and November 2011. Analysis of this erosion event and comparison with previous coastal erosion events in 2006/2007 offered the opportunity to extend the understanding of the time and place of coastal erosion strikes. The swells that drove the erosion hotspots of the 2011 erosion season were relatively low (significant wave heights were between 2 m and 4.5 m) but of long duration. Although swell height was important, swell-propagation direction and particularly swell duration played a dominant role in driving the 2011 erosion event. Two erosion hotspot types were noted: sandy beaches underlain by shallow bedrock and thick sandy beaches. The former are triggered by high swells (as in March 2007) and austral winter erosion events (such as in 2006, 2007 and 2011). The latter become evident later in the austral winter erosion cycle. Both types were associated with subtidal shore-normal channels seaward of megacusps, themselves linked to megarip current heads. This 2011 coastal erosion event occurred during a year in which the lunar perigee sub-harmonic cycle (a ±4.4-year cycle) peaked, a pattern which appears to have recurred on the KwaZulu-Natal coast. If this pattern proves true, severe coastal erosion may be expected in 2015. Evidence indicates that coastal erosion is driven by the lunar nodal cycle peak but that adjacent lunar perigee sub-harmonic peaks can also cause severe coastal erosion. Knowing where and when coastal erosion may occur is vital for coastal managers and planners

Large-scale forcing of the European Slope Current and associated inflows to the North Sea

Drifters drogued at 50?m in the European Slope Current at the Hebridean shelf break follow a wide range of pathways, indicating highly variable Atlantic inflow to the North Sea. Slope Current pathways, timescales and transports over 1988–2007 are further quantified in an eddy-resolving ocean model hindcast. Particle trajectories calculated with model currents indicate that Slope Current water is largely ''recruited'' from the eastern subpolar North Atlantic. Observations of absolute dynamic topography and climatological density support theoretical expectations that Slope Current transport is to first order associated with meridional density gradients in the eastern subpolar gyre, which support a geostrophic inflow towards the slope. In the model hindcast, Slope Current transport variability is dominated by abrupt 25–50?% reductions of these density gradients over 1996–1998. Concurrent changes in wind forcing, expressed in terms of density gradients, act in the same sense to reduce Slope Current transport. This indicates that coordinated regional changes of buoyancy and wind forcing acted together to reduce Slope Current transport during the 1990s. Particle trajectories further show that 10–40?% of Slope Current water is destined for the northern North Sea within 6 months of passing to the west of Scotland, with a clear decline in this Atlantic inflow over 1988–2007. The influence of variable Slope Current transport on the northern North Sea is also expressed in salinity, which declines through the hindcast period, and there is evidence for a similar freshening trend in observational records. A proxy for Atlantic inflow may be found in sea level records. Variability of Slope Current transport is implicit in mean sea level differences between Lerwick (Shetland) and Torshavn (Faeroes), in both tide gauge records and a longer model hindcast spanning 1958–2013. Potential impacts of this variability on North Sea biogeochemistry and ecosystems, via associated changes in seasonal stratification and nutrient fluxes, are discussed

A new approach to projecting 21st century sea-level changes and extremes

Future increases in flooding potential around the world’s coastlines from extreme sea level events is heavily dependent on projections of future global mean sea level (GMSL) rise. Yet, the two main approaches for projecting 21st century GMSL rise—i.e., process-based versus semi-empirical—give inconsistent results. Here, a novel hybrid approach to GMSL projection, containing a process-based thermosteric contribution and a semi-empirical ice-melt contribution, is embedded within a conceptual Earth system model (ESM). The ESM is run 10 million times with random perturbations to multiple parameters, and future projections are made only from the simulations that are historically consistent. The projections from our hybrid approach are found to be consistent with the dominant process-based GMSL projections from the Climate Model Intercomparison Project phase 5 (CMIP5) ensemble, in that our future ensemble-mean projections lie within ±2 cm of CMIP5 for the end of the 21st century when CMIP5-simulated histories are used to constrain our approach. However, when observations are used to provide the historic constraints for our hybrid approach, we find higher ice-melt sensitivity and additional ensemble-mean GMSL rise of around 13–16 cm by the end of the century. We assess the impact of this additional GMSL rise, projected from observation-consistency, on the increase in frequency of extreme sea level events for 220 coastal tide-gauge sites. Accounting for regional effects, we infer a 1.5–8 times increase in the frequency of extreme sea-level events for our higher GMSL projections relative to CMIP5.This work was supported by a UK Natural Environment Research Council (NERC) grants NE/P01495X/1 and NE/N009789/1, and contributes to UK-NERC consortium iGlass (NE/I009906/1). EJR acknowledges support from Australian Laureate Fellowship FL12010005

Large-scale forcing of the European Slope Current and associated inflows to the North Sea

Drifters drogued at 50?m in the European Slope Current at the Hebridean shelf break follow a wide range of pathways, indicating highly variable Atlantic inflow to the North Sea. Slope Current pathways, timescales and transports over 1988–2007 are further quantified in an eddy-resolving ocean model hindcast. Particle trajectories calculated with model currents indicate that Slope Current water is largely ''recruited'' from the eastern subpolar North Atlantic. Observations of absolute dynamic topography and climatological density support theoretical expectations that Slope Current transport is to first order associated with meridional density gradients in the eastern subpolar gyre, which support a geostrophic inflow towards the slope. In the model hindcast, Slope Current transport variability is dominated by abrupt 25–50?% reductions of these density gradients over 1996–1998. Concurrent changes in wind forcing, expressed in terms of density gradients, act in the same sense to reduce Slope Current transport. This indicates that coordinated regional changes of buoyancy and wind forcing acted together to reduce Slope Current transport during the 1990s. Particle trajectories further show that 10–40?% of Slope Current water is destined for the northern North Sea within 6 months of passing to the west of Scotland, with a clear decline in this Atlantic inflow over 1988–2007. The influence of variable Slope Current transport on the northern North Sea is also expressed in salinity, which declines through the hindcast period, and there is evidence for a similar freshening trend in observational records. A proxy for Atlantic inflow may be found in sea level records. Variability of Slope Current transport is implicit in mean sea level differences between Lerwick (Shetland) and Torshavn (Faeroes), in both tide gauge records and a longer model hindcast spanning 1958–2013. Potential impacts of this variability on North Sea biogeochemistry and ecosystems, via associated changes in seasonal stratification and nutrient fluxes, are discussed